Studies on the biosynthesis of proteins in the silkworm, Bombyx mori (L.)

Abstract

The incorporation of [14C]glycine into tissue proteins of injected silkworm larvae was studied, and depends on the stage of development of the larvae. The rate is low during the 4th molt and increases progressively during the 5th larval instar. Just before spinning the rate of incorporation is again low. Incorporation in vitro into silk-gland minces is an aerobic process depending upon the presence of bivalent ions. It is enhanced.by the addition of intermediates of the tricarboxylic acid cycle, particularly by malate, citrate, succinate, fumarate and a-glycerophosphate. A pH 5 extract was prepared by precipitating at pH 5.1 an ultra-centrifugate of brei obtained from the posterior portion of the silk gland and subsequently dissolving the precipitate at pH 7.8. Incorporation of [14C]glycine into this extract is rapid, and is stimulated by Mg2+ ions and either adenosine or guanosine triphosphate. In addition to [14C]glycine, labelled phenylalanine and glutamate are incorporated into the pH 5 extract but at a slower rate; in mixtures, the 3 amino acids are incorporated additively. The [1-14C]glycine-pH 5 extract complex is decomposed by heating at pH 7.8 by treatment with hot trichloroacetic acid, and by the action of ribonuclease. A glycine-activating enzyme has been purified 20-fold by ammonium sulfate precipitation of the pH 5 extract. The purified preparation no longer incorporates [14C]glycine, which indicates that separate enzymes are involved in activation and incorporation.